I've got two home computers with GPU's in them, but they only seem to get GPU work on and off. Is that normal?

When I first started using my GPU's in October/November I was crunching around 20,000 to 30,000 credits a day. But since then I've hardly had any GPU work and my credits have died down to 5,000 to 6,000 a day. Because I've only just started doing GPU work, I'm not sure if there is a problem, or this is normal for GPU crunching.